Revision as of 09:48, 16 June 2011

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How to use EclipseLink Caching

By default EclipseLink uses a shared object cache, that caches a subset of all objects read and persisted for the persistence unit.
The EclipseLink shared cache differs from the local EntityManager cache. The shared cache exists for the duration of the persistence unit (EntityManagerFactory, or server) and is shared by all EntityManagers and users of the persistence unit. The local EntityManager cache is not shared, and only exists for the duration of the EntityManager or transaction.

The benefit of the shared cache, is that once an object has been read, if it is read again using the find operation, the database does not need to be accessed.
Also if the object is read through any Query, it will not need to be rebuilt, and its relationships will not need to be re-fetched.

The limitation of the shared cache, is that if the database is changed directly through JDBC, or by another application or server, the objects in the shared cache will be stale.

EclipseLink offers several mechanism to deal with stale data including:

Refreshing

Invalidation

Optimistic locking

Cache coordination

The shared cache can also be disabled. This can be done using the persistence unit property:

<propertyname="eclipselink.cache.shared.default"value="false"/>

Or, in JPA 2.0 the shared-cache-mode element in the persistence.xml can be used,

<shared-cache-mode>NONE</shared-cache-mode>

Or can be selectively enabled/disabled using the @Cache annotation. Or in JPA 2.0 using the @Cacheable annotation.

EclipseLink also offers several different caching strategies, the configure how many objects are cached, and how much memory is used.

Note, the CacheType NONE on the @Cache annotation should not be used to disable the cache, instead shared should be set to false.

Both of these invalidate all of the objects in the cache. The cache can also be cleared,
clearing the cache can cause object identity issues if any of the cached object is in use, so invalidating is safer.
If you know that none of the cached objects are is use then you can just clear the cache.

Set this attribute to the type (org.eclipse.persistence.annotations.CacheType enumerated type) of the cache that you will be using:

FULL

WEAK

SOFT

SOFT_WEAK

HARD_WEAK

CACHE (not recommended)

NONE (not recommended, use shared=false instead)

CacheType.SOFT_WEAK

optional

eclipselink.cache.type.<ENTITY>

eclipselink.cache.type.default

size

Set this attribute to an int value to define the size of cache to use (number of objects).

100

shared

Indicate whether cached instances should be in the shared cache or in a client isolated cache (see Isolated Client Session Cache).
This allows the shared cache (L2 cache) to be disabled.

true - use shared cache for cached instances;

false - use client isolated cache for cached instances (no L2 cache).

true

optional

expiry

The int value to enable the expiration of the cached instance after a fixed period of time (milliseconds). Queries executed against the cache after this will be forced back to the database for a refreshed copy.

-1 (no expiry)

optional

expiryTimeOfDay

Specific time of day (org.eclipse.persistence.annotations.TimeOfDay) when the cached instance will expire. Queries executed against the cache after this will be forced back to the database for a refreshed copy.

@TimeOfDay(specified=false)

optional

alwaysRefresh

Set to a boolean value of true to force all queries that go to the database to always refresh the cache.

false

optional

refreshOnlyIfNewer

Set to a boolean value of true to force all queries that go to the database to refresh the cache only if the data received from the database by a query is newer than the data in the cache (as determined by the optimistic locking field).

Note: This option only applies if one of the other refreshing options, such as alwaysRefresh, is already enabled.

Note: A version field is necessary to apply this feature.

false

optional

disableHits

Set to a boolean value of true to force all queries to bypass the cache for hits, but still resolve against the cache for identity. This forces all queries to hit the database.

false

optional

coordinationType

Set this attribute to the cache coordination mode (org.eclipse.persistence.annotations.CacheCoordinationType enumerated type).

CacheCoordinationType.SEND_OBJECT_CHANGES

optional

Caching in Clustered Environments

Caching in a clustered environment can have issues as changes made on one server will not be reflected on objects cached in other servers.
For read-only objects, this is not an issue, but for objects that are frequently updated this can be an issue.

EclipseLink offers several solutions to this issue.

The cache can be disabled for the classes that frequently change.

Cache coordination can be used to broadcast changes between the servers in the cluster to update of invalidate changed objects.

Cache invalidation based on time-to-live or time-of-day.

Optimistic locking will prevent updates to stale objects, and trigger the objects to be invalidated in the cache.